1. Field of the Invention
Embodiments of the present invention generally relate to ion spectrometers, and more specifically to, a system, method, and apparatus for greater control over an ion spectrometer drift chamber.
2. Description of the Related Art
Ion mobility spectrometers have many applications, including security applications where the ion mobility spectrometer is used to search for unwanted substances (e.g., to identify explosives, narcotics, and other contraband).
Some prior art ion spectrometers acquire a sample by wiping a woven or non-woven fabric trap across a surface that is to be tested for molecules of interest. Other prior art ion spectrometers create a stream of gas adjacent the surface to be tested for the molecules of interest or rely upon an existing stream of gas.
FIG. 1 depicts a typical prior art ion mobility spectrometer 100. The ion spectrometer 100 includes a housing 102 (also known as a “bottle” 100); a gas inlet 106 (for receipt of a dopant (i.e., air in combination with ammonia and/or methylene chloride); a semi-permeable membrane 104; an inlet tube 118; an ionization chamber 114; radioactive source 116; electrodes 122, 124, 126, 128, 130, and 132; a drift region 112; an anode screen grid 134; an anode 136; and a gas exhaust 110.
When there is a chemical that needs to be identified, a sample of the chemical is taken. For example, a swab is wiped on an object containing the questionable chemical. The swab is placed against the semi-permeable membrane 104. The swab is then heated and the chemical(s) (e.g., explosives, narcotics, and the like) are turned into a vapor. The vapor permeates the membrane 104 while the membrane 104 helps to keep out contaminants (e.g., water).
An inlet tube 106 provides an inert gas (which includes air and a dopant (i.e., ammonia and/or methylene chloride)), which forces the vapor towards an ionization chamber 114. While in the ionization chamber 114, the vapor is exposed to a radioactive material 116 (i.e., nickel 63 or tritium). The radioactive material 116 bombards the vapor molecules with beta-particles and creates ions (i.e., charged molecules) from the vapor molecules.
A population of the ions builds up in the ionization chamber 114. An ion grid 120 separates the charged molecules from the drift region 112. The drift region 112 also includes a plurality of field-defining electrodes 122, 124, 126, 128, 130, and 132; and an anode screen grid 134 at the end of the drift chamber opposite the ionization chamber 114. Electrode 122 also includes a perforated ion grid 120 that, at the appropriate time, allows ions to pass through the perforations.
Electrodes 122, 124, 126, 128, 130, and 132 are each shaped like a disk. Because of their shape, electrodes 122, 124, 126, 128, 130, and 132 are referred to herein as “disk electrodes.” “Disk shaped” as used herein is generally defined as a shape similar to a circular plate having a hole therethrough. The disk shape of the electrodes protrudes into the drift region 112 and has spaces there-between.
During manufacture of a spectrometer unwanted substances (e.g., cutting oil) can remain in the spectrometer. These unwanted substances often collect in the spaces between the disk shaped electrodes. In addition, after the spectrometer has analyzed a substance of interest, the analyzed substance of interest is no longer needed in the spectrometer and is considered an unwanted substance with respect to tests performed on subsequent substances of interest. The spaces between the disk shaped electrodes provide areas where the unwanted substances are trapped in the drift region. A “contaminant” as used herein is generally defined as any unwanted substance.
The impedance of the flow of ions can cause multiple problems. For example, during fabrication of the ion spectrometer, the spectrometer must be “burned in.” The length of time for the burn in process is, in part, dependant upon the shape and configuration of the electrodes. The duration of the burn in time slows the manufacturing process. Other examples, a longer time to flush ions out of corners formed between the electrodes 122, 124, 126, 128, 130, and 132; and a non-uniform electric field (e.g., eddy currents) produced in the drift region 112.
After the ions have built up in the ionization chamber 114, a voltage is varied at the G1 electrode 122 to accelerate the ions through the ion grid 120 and into the drift region 112. The ions strike the anode 136 (also know as the collector electrode).
The anode 136 is coupled to an amplifier (not shown). The amplifier amplifies signals (i.e., ion currents) received by the anode 136. When a change in ion current is detected, the time that respective ions take to travel through the drift region 112 is measured. Larger ions move through the drift region 112 slower than smaller ions. The time taken to travel through the drift region 112 is used to derive the identity of the ions.
As the ions are analyzed, they are flushed out of the drift region 112 through a gas exhaust 110 and into a pump (not shown) and dryer (also not shown) for recycling of the dopant.
There is a need in the art for an improved electrode configuration that avoids the shortcomings and drawbacks of prior art systems and methodologies (e.g., which allows a shorter burn in time; a more uniform electric field; and easier flushing of contaminants).